Gabrielle Giffords, the US congresswoman who was shot in the head during an attack at a constituency event in Arizona in which six other people were killed last January, has announced that she is stepping down.

Speaking slowly and haltingly, the Democrat politician said in a video message released on YouTube on Sunday that she had more work to do on her recovery.

"Arizona is my home – always will be," Giffords said on the video, which was interspersed with footage of life in the state, moments from her political career and images of her alongside her husband.

"A lot has happened over the past year. We cannot change that but I know, on the issues we fought for, we can change things for the better: jobs, border security, veterans," she said. "We can do so much more by working together. I don't remember much from that horrible day, but I will never forget the trust you placed in me to be your voice.

"Thank you for your prayers and for giving me time to recover. I have more work to do on my recovery, so to do what is best for Arizona I will step down this week. I'm getting better every day. My spirit is high. I will return and we will work together for Arizona and this great country."

Jared Loughner, the man accused of the shootings on 8 January last year, pleaded not guilty to 49 charges and is being held in a mental health facility. He has been diagnosed with bipolar disorder, and is being treated in an effort by authorities to make him mentally fit for trial.

A federal judge and an aide to Giffords were among those killed in the shootings, which prompted a debate about highly-charged rhetoric in political campaigns.

In the months since she was shot, Giffords, 41, has learned how to walk and speak again following injuries to the left side of the brain, the part that controls speech and communication.

Last August, she made a dramatic appearance in the House of Representatives, when she unexpectedly arrived to vote for an increase in the US debt limit.

Weeks before, she had watched the launch from Cape Canaveral of the last mission by the space shuttle Endeavour, which was commanded by her husband, Mark Kelly. The veteran astronaut, who made four trips into space aboard the space shuttle, retired in October.

Democratic officials had held out hope for months that she might recover sufficiently to run for re-election. She had until May to decide on putting her name forward for the November elections, while more than $800,000 was raised for a potential campaign.

Barack Obama praised Giffords as an "inspiration" and "the very best of what public service should be." He said the congresswoman and her husband had "taught us the true meaning of hope in the face of despair" and "determination in the face of incredible odds".

The Republican speaker of the House, John Boehner, said he saluted Giffords "for her service and for the courage and perseverance she has shown in the face of tragedy".

The House Democratic leader Nancy Pelosi of California said that "since the tragic events one year ago, Gabby has been an inspiring symbol of determination and courage to millions of Americans".